In recent years, with a rise in awareness of environmental problems, solar cells have been receiving attention as a clean energy source. In general, a solar cell module constituting a solar battery has a structure in which a transparent front board, a surface side sealer sheet, a solar cell element, a back surface side sealer sheet, and a back surface protecting sheet are laminated in this order when viewed from the side of the light-receiving surface, and it has a function of generating electricity from sunlight incident on the solar cell element.
A plurality of solar cell elements, which generate electricity within a solar cell module, are provided within a solar cell module, and they are configured to generate necessary voltage and current by being connected in series-parallel combination. To wire the plurality of solar cell elements within the solar cell module, for example, a collector sheet for solar cells is used, in which metal foil to be a wiring pattern is laminated on the surface of a resin sheet as a base (see, Patent Document 1). Then, the wiring pattern composed of a metal foil and the output electrodes of the solar cell element are electrically joined to each other by soldering.
To form the wiring pattern on the surface of the resin sheet as a base for the collector sheet for solar cells, the same method as that used to make a printed wiring board, for example, can be used; that is, a conductive base composed of a metal foil is first laminated onto the whole surface of the base, and then the conductive base is etched into the desired wiring pattern by photolithography.
However, the surface of the copper foil generally used as the aforementioned conductive base is very easily oxidized. Furthermore, the surface of the oxidized copper foil has extremely low wettability for solder. For such reasons, the surface of the copper foil to be a wiring pattern needs to be subjected to rust inhibiting processing.
From this point of view, on a printed wiring board, a rust inhibiting treatment using an organic rust inhibitor, such as a method of coating a rust inhibiting coating film onto the surface of a copper foil to be a wiring pattern (see, Patent Document 2) and a method of forming a coating film having a complex structure formed with copper on the surface of a wiring pattern (see, Patent Document 3), is performed on the surface of the conductive base to be a wiring pattern. Since a printed wiring board obtained after a rust inhibiting treatment has inhibited oxidation on the surface of the wiring pattern, good wettability for solder is maintained, and thus it is desirable from the viewpoint of having reliable installation of an electronic component onto the printed wiring board by soldering.
However, the collector sheet for solar cells is put to use in a solar cell element-joined state while being exposed to sunlight for a long period of time like several tens of years. For such reasons, when a rust inhibiting treatment using an organic rust inhibitor, like in the case of a printed wiring board, is performed, the organic rust inhibitor itself or a compound generated by decomposition of the inhibitor exerts a bad influence on the solar cell element during use for a long period of time, and thus may cause performance degradation of the solar cell module.
The conductive base for forming a wiring pattern of a collector sheet for solar cells requires a rust inhibiting treatment that does not use an organic rust inhibitor. As such a rust inhibiting treatment, treatment forming a rust inhibiting layer on a copper foil by plating containing chrome and zinc, is performed (see, Patent Document 4).
Alternatively, a copper foil having a chrome-free property and also a rust inhibiting property by forming a surface treated layer composed of a nickel-zinc alloy in which the content ratio of nickel is limited to 50% or more on the surface has been also suggested (see, Patent Document 5).    [Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2007-081237    [Patent Document 2] Japanese Unexamined Patent Application, Publication No. H9-326549    [Patent Document 3] Japanese Unexamined Patent Application, Publication No. H6-006018    [Patent Document 4] Japanese Unexamined Patent Application, Publication No. 2000-178787    [Patent Document 5] Japanese Unexamined Patent Application, Publication No. 2010-202891